This invention relates to a system and a method for measuring the core body temperature of a patient. More particularly, the present invention relates to a system and a method for deriving the core body temperature from measurements of respired gas temperature.
The measurement of body temperature during surgery and in other critical care situations is essential for the appropriate clinical management of any patient. Hypothermia during general anesthesia is a common occurrence especially in pediatric and elderly patient populations which can be associated with undesirable consequences including reduced drug metabolism and excretion, hemodynamic instability and delayed awakening. During the recovery period following general anesthesia, the hypothermic patient must attempt to restore normal body temperature, commonly through shivering thermogenesis and peripheral vasoconstriction to conserve heat. However, shivering can increase the body's oxygen consumption by up to 500 percent, leading to the possibility of decreasing the oxygen available to the heart and brain. Additionally, vasoconstriction decreases peripheral blood flow which may interfere with the normal healing process. In view of complications such as these, it is often desirable to measure and to monitor the core body temperature of a patient.
The most accurate sites for measuring core temperature include the pulmonary artery, the tympanic membrane, the distal esophagus and the nasopharynx. However, not all of these sites are ideal. Access to the pulmonary artery site requires pulmonary artery catheterization, an invasive procedure that is usually not performed for temperature monitoring alone. Temperature measurement at the tympanic membrane requires very careful probe placement and risks damage to the tympanic membrane. The esophageal site is safe, but the temperature probe must be carefully positioned to avoid cooling by inspired gases in the adjacent trachea. Nasopharyngeal temperature measurements are very prone to error as a result of movement of the temperature probe and cooling effects of respired gases.
A measurement site within the trachea would be most convenient, since nearly every patient undergoing general anesthesia has an endotracheal tube placed in the trachea to protect the airway and to enable breathing (or ventilation) during anesthesia. Unfortunately, systems and methodologies for measuring temperature within the trachea provide inaccurate temperature measurements as the result of cooling by inspired gases.